<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <!--
    左上、右上、右下、左下
    圆圈:  圆角 = 半径！(盒子的半径)
     -->
    <style>
        div {
            width: 100px;
            height: 100px;
            border: 10px solid green;
            border-radius: 120px 120px 0 0;
        }
    </style>
</head>
<body>

<div>

</div>

</body>
</html>